1. Field of the Invention
The invention relates to a projection objective for short wavelengths, in particular for wavelengths λ<157 nm, having a number of mirrors that are arranged positioned precisely in relation to an optical axis, and wherein the mirrors have multilayer coatings. The invention also relates to a projection exposure apparatus for EUV lithography as well as an X-ray optical subsystem for X-rays of wavelength λR.
2. Description of the Related Art
Projection objectives that are used in the extreme ultraviolet region are transradiated with soft X-radiation. Here, the wavelength region is at 10 to 30 nm. The materials previously capable of use for the optics are opaque to the extreme UV light used in the case, the imaging beams no longer being guided through lenses by refraction, but it being possible only to make use of mirrors. The mirrors used should have as high a reflectivity as possible in the EUV region. Such mirrors comprise a substrate that is provided with a multilayer system, also termed multilayer. This permits the implementation of mirrors of high reflectivity in the X-ray region when the incidence is not grazing, that is to say the implementation of normal incidence mirrors. Layer systems, for example with Mo/Si (molybdenum/silicon), Mo/Be (molybdenum/beryllium), MoRu/Be layer stacks with 40 to 100 layer pairs can be used for such multilayer systems, it being possible thereby to achieve peak reflectivities of 70 to 80% in the EUV region with λ=10 to 20 nm. Different layer systems can be used depending on the wavelength of the light to be reflected.
A high reflectivity of the layer stacks can be achieved by in-phase superposition and constructive interference of the partial wavefronts reflected at the individual layers. The layer thicknesses should, moreover, typically be controlled in the sub-Ångström region (<0.1 nm) in this case.
Multilayer-coated X-ray mirrors are operated near normal incidence, and are always preferred to those mirrors, coated with simpler layers, with grazing incidence whenever there is a requirement for a high imaging quality owing to slight aberrations, preferably in imaging systems. The reflectivity of grazing incidence mirrors can, however, be further increased nevertheless by applying a multilayer.
For mirrors, in particular X-ray mirrors, of a EUVL projection objective or projection system, the properties described below should be fulfilled at the same time; these ensure that the structures can be transferred onto the wafer in a fashion faithful to the mask, and enable a high contrast of the image and a high reflectivity of the reflective layer.
A good figure, that is to say errors in the low spatial frequency region, could be mentioned as first property. This is to be understood in general as structural sizes between 1/10 over the beam cross sections assigned by the individual pixels, up to a free diameter of the mirror. This means that the errors have lateral extents of an order of magnitude from a millimeter up to a number of decimeters. Such errors lead to aberrations, and thereby reduce the imaging quality and restrict the resolution limit of the overall system.
Furthermore, the X-ray mirrors should have a slight roughness in the MSFR (mid spatial frequency roughness) region (middle spatial frequency region). Such spatial wavelengths typically occur in the region between approximately 1 μm and approximately 1 mm, and lead to scattered light within the image field, and therefore to contrast losses in the imaging optics.
Requisite conditions for achieving high reflectivities are sufficiently low layer and substrate roughnesses in the so-called HSFR (high spatial frequency roughness) region. Depending on the point of view, the HSFR region leads to light losses by scattering outside the image field of the optics, or owing to interference in the microscopically in-phase superposition of the wavetrain components. The relevant spatial wavelength region is bounded above by the criterion of scattering outside the image field and, depending on application, is situated at EUV wavelengths in the region of a few μm. In general no bound is defined at the high-frequency limit. It is thereby possible to specify a reasonable guideline in the range of half the wavelength of the incident light, since in accordance with prior knowledge even higher spatial frequencies of the incident photons can no longer be seen. The HSFR can be measured by the known atomic force microscopes (AFM) which have the requisite lateral and vertical resolution. It is necessary in projection optics to control both figure and the MSFR and the HSFR to within a few Ångström rms (root mean square).
Furthermore, use should be made for the X-ray mirrors of materials that have the smallest coefficient of thermal expansion possible such as, for example ZERODUR® or ULE®. It is thereby possible to keep the surface shape of the mirror stable even during operation under thermal loads. Even monocrystalline silicon could be used as a carrier, since it permits very low roughnesses. Furthermore, in silicon the higher coefficient of thermal expansion can be partially compensated by the substantially higher thermal conductivity and a suitable cooling. However, silicon has a mechanical anisotropy, and can generally be used only for small mirror sizes, owing to the required monocrystallinity. Moreover, the comparatively high price of the monocrystalline material is a substantial disadvantage. Consequently, silicon will be used only in the event of very high thermal loads, for example in illumination systems.
It has emerged that consideration has so far been given only to suitable glass ceramic materials, for example Schott: ZERODUR®, Ohara: CLEARCERAM-Z®, or also amorphous titanium silicate glasses, for example Corning: ULE®, for such mirrors, since these materials have a coefficient of thermal expansion (CTE) that can be made to vanish at a specific temperature that is also denoted as zero crossing temperature (ZT). ZERODUR® is a registered trademark of Schott AG of Mainz, Germany; CLEARCERAM-Z® is a registered trademark of Kabushiki Kaisha Ohara of Kanagawa, Japan, and ULE® is a registered trademark of Corning Incorporated of Corning, N.Y. In the case of finite, local and global deviations of the operating temperature from the zero crossing temperature, the coefficient of thermal expansion does not however, vanish completely, and a deformation of the surface therefore results. The tolerances for these deformations are approximately 100 nm for global, homogeneous deformations of the mirror, and in the region of 50 pm-200 pm for local spatially varying deformations. It has emerged from the conduct of simulations that, in particular, distortion errors of the projection objective, the optical components contained therein consisting either only of ULE® or only of ZERODUR®, react so sensitively to thermal loads that they must be compensated during operation by means of expensive manipulators and with the acceptance of dead times.
When the materials currently used are employed, the system quality that can be achieved with regard to X-ray optics is greatly impaired in different ways.
Reference may be made as regards the projection optics for EUV lithography and the X-ray optical components used to DE 100 37 870 A1 and to U.S. Pat. No. 6,353,470 B1, the statements of which are also fully incorporated into the present application.
Titanium silicate glass, also known as ULE®, is specifically specified in WO 01/08163 A1 for projection objectives in EUV lithography. That document describes a projection lithography method for producing integrated circuits and generated patterns with extremely small object dimensions. An illumination subsystem illuminates a mask or a reticle with X-radiation. A projection subsystem has reflective, multilayer coated titanium silicate glasses that have a faultless surface. In the inventive method, the titanium silicate glasses are heated by means of the X-radiation to an operating temperature, the level of titanium doping substance preferably being regulated in such a way that the glass has a coefficient of thermal expansion that is centered on zero at the operating temperature. The titanium silicate glass specified here therefore has a variation of ≦10 ppb in the coefficient of thermal expansion.